Neocities.org
After some searching on where the images in this link (https://megamitensei.fandom.com/wiki/Persona_20th_Anniversary_Festival) came from, I finally found it! They were created to commemorate Persona's 20th anniversary and were exhibited in the 3331 Arts Chiyoda Museum (https://personacentral.com/persona-20th-fes-anniversary-event-venue-pictures/)!
It's also unfortunate that I can't get them in a higher quality, but at least I know their source. No wonder I couldn't find anything about them on SauceNao or TinEye, they were never posted online in the first place!
After finding this (https://twitter.com/uedas10/status/812085012111597569/) I'm starting to think SauceNao isn't particularly good at sourcing non-Pixiv art. It's hard to find these kinds of things because: 1. Often the artists don't tag their work and 2. Pieces made for a specific event in 2016 and for a popular series oft get overlooked after the event is over, and drowned out by 5 years' worth of fanart.